NO317897B1 - Apparatus and method for monitoring the autonomic nervous system of a sedentary patient. - Google Patents

Description

Technical area

The invention generally relates to medical technology, and in particular to a method and apparatus for monitoring the autonomic nervous system in a sedated patient. More specifically, the invention relates to a method and an apparatus for simultaneously indicating the possible state of pain/discomfort and the possible state of alertness in a sedated patient, based on measurements of the patient's skin conductivity.

Background for the invention

Within the area of medical technology, there is a problem of producing physical measurements that represent the activity in the autonomic nervous system of an individual, i.e. the part of the nervous system that is beyond the control of the will. In particular, there is a special need to monitor the autonomic nervous system in a sedated, non-verbal patient, e.g. a patient under anesthesia or an artificially ventilated patient, to detect whether the patient needs more analgesics as a result of pain/discomfort stimuli or hypnotics as a result of wakefulness stimuli.

Analgesics are given to avoid pain/discomfort, and hypnotics are given to avoid wakefulness. Pain/discomfort can induce wakefulness, and wakefulness rarely induces pain/discomfort. If hypnotics are given to a patient who feels pain/discomfort, the stress activation may be reduced, but the patient may still feel pain/discomfort. There is therefore a need to provide a monitoring system that can provide information on whether the stress activation found in sedated patients is due to pain/discomfort stimuli or wakefulness stimuli.

Both patients under anesthesia and patients who are artificially ventilated are treated with analgesics and hypnotics. Currently, stress activation is monitored for these patients by an increase in blood pressure and heart rate. Blood pressure and heart rate are influenced by many other factors than the need for analgesics or hypnotics, such as blood circulation changes that occur with heart disease, hypertension, lung disease, anemia, blood loss and blood poisoning, to name a few.

1-2% of patients under anesthesia feel pain during surgery. The development in anesthesia medication is to provide both hypnotics and analgesics with a very short half-life. It will then be even more important to monitor the patient's need for analgesics and hypnotics.

Tests have shown that the skin's conductivity changes as a time-varying signal which, in addition to a basal, slowly varying value (the so-called basal level or the average conductivity level over a certain interval), also has a component consisting of spontaneous waves or fluctuations, in which characteristics by these fluctuations, such as e.g. their frequency and amplitude are factors that are correlated with the experience of pain in the target object (the patient). Measuring and analyzing the characteristics of these fluctuations is a known method for providing information relating to the activity of the sympathetic nervous system, including the effect of pain.

Related background technology

WO 00/72751 A1 describes a method and an apparatus for monitoring the autonomic nervous system of an individual. In accordance with the method, a measurement signal is provided which expresses the conductivity of at least one area of the individual's skin. The measured signal values are stored at discrete times in a time window. Furthermore, in accordance with the method, an analysis of the measurement signal is carried out in the time window, including calculation of the amplitude and the number of fluctuation peaks in the conductivity signal in the said time window. From the aforementioned characteristics at the peaks, an output signal is determined which indicates the state of pain in the individual.

This procedure provides an indication of pain but no indication of alertness. When the prior art method is used to monitor a sedated patient, it will provide an indication of a lack of analgesics, but it will not provide an indication of a lack of hypnotics due to wakefulness.

WO 85/00785 A1 describes an apparatus for monitoring the alertness or concentration of a person such as a long distance driver. Volar skin resistance is monitored, and if the resistance increases more than a predetermined value, a stimulus, e.g. an audible tone in a headphone is emitted to alert the driver and to increase his concentration.

Although this simple way of monitoring the skin resistance value may provide an indication of the person's alertness, it would not provide an indication that is sufficiently reliable for use in monitoring alertness for a sedated patient. In particular, this prior art solution does not provide simultaneous indications of pain/discomfort and alertness.

Summary of the Invention

One purpose of the present invention is to provide a method and apparatus for monitoring the autonomic nervous system of a sedated patient, which indicates a state of pain/discomfort in the patient, and which also provides an indication of alertness in the patient.

Another purpose of the invention is to provide such a method and an apparatus which makes use of the measurement of skin conductivity variations as a result of emotional sweating.

Yet another purpose of the invention is to provide such a method and apparatus which provides reliable output indications. 1 in accordance with the invention, the above objectives are achieved by means of the method as stated in the attached claim 1 and by means of the apparatus stated in the attached claim 7.

Further advantages and characteristics of the invention are set out in the independent claims.

Emotional sweating provides a more accurate and precise means of monitoring the need for hypnotics (after wakefulness stimuli) and analgesics (after pain-pleasure stimuli) than measurements of blood pressure and pulse. Unlike blood pressure and heart rate, emotional sweating is not affected by circulatory changes that occur with heart disease, hypertension, lung disease, anemia, blood loss, and blood poisoning, to name a few. Furthermore, when the measurement of blood pressure and pulse changes, healthcare personnel are unable to know the patient's need for more analgesics or hypnotics.

Brief description of the drawings

The apparatus and method according to the invention will now be described in more detail with reference to the attached drawings, where fig. 1 is a block diagram of an apparatus in accordance with the invention, and FIG. 2 is a flowchart illustrating a method in accordance with the invention,

fig. 3a-c are three graphs each illustrating a time series of skin conductance measurements for a sedated patient, exposed to neither wakefulness nor pain-pleasure stimuli,

fig. 4 is a graph illustrating a time series of skin conductance measurements for a sedated patient exposed to pain/discomfort stimuli, and .

fig. 5 is a graph illustrating a time series of skin conductance measurements for a sedated patient exposed to wakefulness stimuli.

Detailed description of the invention

Fig. 1 illustrates a block diagram for a preferred embodiment of an apparatus in accordance with the invention. The device is specially designed for the simultaneous detection of pain/discomfort reaction and alertness in a sedated patient. On an area 2 of the skin on a body part 1 of the patient, sensor means 3 are placed for measuring the conductivity of the skin. The body part 1 is preferably a hand or a foot, and the area 2 of the skin on the body part 1 is preferably the inside of the hand (in the palm) or the underside of the foot (under the sole of the foot). The sensor means 3 comprise contact electrodes where at least two electrodes are placed on the skin area 2.1 a preferred embodiment, the sensor means 3 consists of three electrodes: a signal electrode, a measuring electrode and a reference voltage electrode, which ensures a constant application of voltage across the stratum corneum (the surface layer of the skin) below the measuring electrode. The measuring electrode and the signal electrode are preferably placed on the skin area 2.

The reference voltage electrode can also be placed on the skin area 2, but it is preferably placed in a place nearby, suitable for the measuring arrangement in question.

In a preferred embodiment, an alternating current is used to measure the conductivity of the skin. The alternating current advantageously has a frequency in the range of up to 1000 Hz, corresponding to the range where the skin's conductivity is approximately linear. A frequency should be chosen that ensures that the measurement signal is affected to the least possible extent by interference from e.g. the mains frequency. In a preferred embodiment, the frequency is 88 Hz. A signal generator operating at the specified frequency supplies a signal current to the signal electrode.

In the case of alternating current, the conductivity is identical to the real part of the complex admittance, and therefore not necessarily identical to the inverse value of the resistance. An advantage of using alternating current instead of direct current in conductivity measurements is that in this way the individual effects of the skin's electrical polarization properties on the measurements are avoided.

The resulting current through the measuring electrode is fed to a measuring converter 4. This comprises a current-to-voltage converter, which in a preferred embodiment is a transresistance amplifier, but which in its simplest form can be a resistance, which converts the current from the measuring electrode into a voltage.

The measuring converter further comprises a decomposition circuit, preferably in the form of a synchronous rectifier, which decomposes the complex admittance into a real part (the conductance; conductivity) and an imaginary part (the susceptance). However, it is sufficient if the decomposition circuit only comprises means for diverting the conductivity. The synchronous rectifier multiplies the measured voltage with the voltage from the signal generator. The two signals are in phase. After multiplication, the result conforms to the cosine (2u) equation, where the result is a DC component and a component with frequency 2u. In the preferred embodiment, this will be 176 Hz. In the preferred embodiment, this synchronous rectifier is realized as an analog circuit with the required accuracy.

The measuring converter 4 can also include amplifier and filter circuits. In the preferred embodiment, the measurement converter contains low-pass filters, both at the input and at the output. The purpose of input low-pass filters is to attenuate high-frequency noise, e.g. that coming from other medical equipment, and also to serve as an anti-collapse filter to avoid high frequency components being received by subsequent time discretization circuits. The output low-pass filter should attenuate the 2u components arising from the multiplication operation in the synchronous rectifier, so that only the near-DC signal is used for further processing.

When choosing components and design details, the measuring converter is also designed with a view to achieving a high sensitivity and a low noise level.

The control unit 5 comprises a time discretization unit 51 for time discretization of the signal from the measurement converter. The time discretization takes place at a sampling rate which can advantageously be in the order of 20-200 samples per second. The control unit further comprises an analog-to-digital converter 52, which converts measurement data into digital form. The choice of circuits for time discretization and analog-to-digital conversion involves technical decisions suitable for a person skilled in the art. In the preferred embodiment, the time discretization is performed in an integrated circuit combining oversampling, filtering and discretization.

The control unit can advantageously include further analogue and possibly also digital inputs (not illustrated), in addition to the input from the measuring converter 4.1 in this case the control unit 5 can either be provided with a plurality of analogue-digital converters 52, or it can use various multiplexing techniques which is well known to those skilled in the art to increase the number of analog inputs. These additional analog inputs can e.g. be arranged for further electrodermal measurements, or for other physiological measurements that can advantageously be carried out simultaneously or in parallel with the electrodermal measurement, such as temperature, pulse, ECG, respiration measurements, oxygen saturation measurements in the blood, or EEG (bispectral index).

The control unit 5 also comprises a processing unit 53 for processing the digital measurement data, storage means in the form of at least one storage for storing data and programs, illustrated as a non-volatile memory 54 and a read/write memory 55. The control unit 5 further comprises an interface circuit 61, which provides output signals 71, 72. Preferably, the control unit 5 further comprises a further interface circuit 81, which is further connected to a display unit 8. The control unit 5 can advantageously also comprise a communication port 56 for digital communication with an external unit, such as a personal computer 10. Such communication is suitable for downloading and changing the program that is kept stored in the memory 54, 55 in the control unit, or for adding or changing other data that is kept stored in the memory 54, 55 in the control unit. Such communication is also suitable for reading out data from the memory 54, 55 in the device, thereby enabling them to be transferred to the external computer 10 for further, subsequent analysis or storage. A communication port 56 in the control unit will advantageously be designed in accordance with the needs for equipment security for patients, as described in more detail below.

In a preferred embodiment, the non-volatile memory 54 comprises a read storage in the form of programmable ROM circuits, which contains at least one program code and permanent data, and the read/write memory 55 comprises a read and write storage in the form of RAM circuits , for storing measurement data and other temporary data.

The control unit 5 also comprises an oscillator (not shown), which supplies a clock signal to control the processing unit 53. The processing unit 53 also contains timing means (eng.: timing means) (not shown), in order to . provide an expression for the current time, for use in analysis of the measurements. Such timing means are well known to those skilled in the art, and are often incorporated into microcontrollers or processor systems that those skilled in the art will find suitable for use with the present invention.

The control unit 5 can be realized as a microprocessor-based unit with connected input, output, memory and other peripheral circuits, or it can be realized as a microcontroller unit in which some or all connected circuits are integrated. The time discretization unit 51 and/or the analog-to-digital converter 52 can also be included in such a unit. The choice of a suitable form for the control unit 5 includes decisions that are suitable for a person skilled in the art.

An alternative solution is to realize the control unit as a digital signal processor

(DSP).

The control unit 5 is arranged to read time-discrete and quantized measurements of the skin conductivity from the measuring transducer 4, preferably by means of an executable program code which is stored in the non-volatile memory 54 and which is executed by the processing unit 53. It is further arranged to put the measurements in capable of being stored in the read and write memory 55. By means of the program code, the control unit 5 is further arranged to analyze the measurements in real time, i.e. simultaneously or in parallel with the execution of the measurements. 1 this connection shall simultaneously or parallel be understood to mean at the same time or parallel for practical purposes, considered in connection with the time constants inherent in the nature of the measurements. This means that input, storage and analysis can be performed in separate time intervals, but in this case these time intervals, and the time between them, are so short that the individual actions appear to occur simultaneously.

The control unit 5 is further arranged to identify an average value for the discrete, quantized measurement signal during a time interval, by means of a program code part which is stored in the non-volatile memory 54, and which is executed by the processing unit 53.

The control unit 5 is further arranged to identify the fluctuations in the time-discrete, quantized measurement signal, by means of a program code part which is stored in the non-volatile memory 54, and which is executed by the processing unit 53. The control unit 5 is further arranged to count or calculate the number fluctuation peaks in the time-discrete, quantized measurement signal during a time interval, by means of a program code part which is stored in the non-volatile memory 54 and which is executed by the processing unit 53.

The processing unit 53, the memories 54, 55, the analog/digital converter 52, the communication port 56, the interface circuit 81 and the interface circuit 61 are all connected to a bus unit 59. The detailed construction of such a bus architecture for designing a microprocessor-based instrument is considered well known to those skilled in the art the area. '

The interface circuit 61 is a digital gate circuit which diverts digital output signals 71, 72 from the processing unit 53 via the bus unit 59 when the interface circuit 61 is addressed by the program code executed by the processing unit 53.

The first digital output signal 71 indicates that the analysis of the skin conductivity measurement has detected that a state of pain/discomfort has occurred in the patient. The second output signal 72 indicates that a state of wakefulness has occurred in the patient.

In a particular application of the invention, the warning signals 71, 72 or another signal derived from the processing means in the analysis of the skin conductivity measurements can be used to control an automatic administration of medication to the patient. In particular, the administration of an analgesic medication may be controlled by the first signal 71 indicating pain/discomfort, and the administration of a sleep-inducing medication or a hypnotic may be controlled by the second signal 72 indicating wakefulness. Each of the signals can e.g. used to control a device for the intravenous supply of medication. In this case, the invention will form part of a feedback loop for controlling the activity in the patient's autonomic nervous system.

In a preferred embodiment, the display means 8 consist of a screen for graphical visualization of the conductivity signal, and a digital display for presenting the frequency and amplitude of the measured signal fluctuations. The display units are preferably of a type with low power consumption, such as an LCD screen and LCD display. The display means can be separate or integrated into one and the same unit.

The device further comprises a power supply unit 9 to supply operating power to the various parts of the device. The power supply can be a battery or a mains supply of a known type.

The device can be advantageously adapted to meet the requirements relating to hospital equipment, which ensures patient safety. Such safety requirements are relatively easy to meet if the device is battery powered. If, on the other hand, the device is mains-powered, the power supply must satisfy special requirements, or requirements are made regarding a galvanic separation between the parts of the device (e.g. battery-powered), which are safe for the patient, and the parts of the device which are unsafe for the patient. If the device is to be connected to external equipment, which is mains powered and unsafe for the patient, it is necessary that the connection between the device, which is safe for the patient and the unsafe external equipment, is galvanically isolated. Galvanic separation of this type can advantageously be achieved by means of an optical separation. Safety requirements for equipment close to the patient and solutions to fulfill such requirements in a device such as in the present invention are well known to professionals in the field.

Fig. 2 illustrates a flow chart for a method of controlling a warning signal in an apparatus for monitoring the autonomic nervous system of a sedated patient, and in particular for detecting pain/discomfort and alertness.

The procedure starts with reference number 31.

The first two process steps 32 and 33 are initial steps, which establish initial values for use in the remaining, repeated process steps.

In the first step 32, a skin conductance signal or EDR (electrodermal response) is measured, which is time quantized and converted to digital form using the equipment described with reference to FIG. 1. An initial time series of some duration, typically a period of 20 sec., containing skin conductance data, is collected during this step. With a sampling rate of 20-200 samples per second, the time series can contain 400-4000 samples.

This time series is then analyzed. In step 33, an average conductivity level or basal level during the initial time series is calculated. This initial mean conductivity value is stored and used as the first "previous value" during the first execution of the comparison step 40 below.

In step 35, a skin conductivity signal is again measured, time quantified and transformed into digital form using the equipment described with reference to fig. 1. A time series of some duration, typically a 20 sec period, containing skin conductance data is collected during this step.

This time series is then analyzed. In step 36, an average conductivity level or basal level is calculated throughout the current time series. This initial average conductivity value is stored and used as the current average conductivity value during execution of the comparison step 40 below. 1 step 37, the number of fluctuation peaks in the conductivity signal is calculated throughout the current time series. This is done by detecting local peaks or local maximum values and/or by detecting local bottoms or local minimum values. Although the following detailed description refers to the detection of peaks, those skilled in the art will recognize that the detection of troughs can be performed in a corresponding manner.

The existence of a peak is established if the derivative of the signal changes sign during a short period in the time interval. The derivative of the signal is calculated as the difference between two consecutive sample values. In addition, it is possible to use a simple digital filter that requires two or more consecutive sign changes to be observed before the sign change is accepted.

In calculation step 37, it may be necessary to establish further criteria for when a peak is to be considered valid. In its simplest form, such criteria can be based on the fact that the signal, in addition to the sign change of the derivative, must exceed an absolute limit to be able to be considered a peak. A recommended such limit value for conductivity is 0.02 uS.

Alternatively or additionally, it is advantageous to base the criteria on the fact that

the signal really has formed a peak that has persisted for some time. The criteria can also be based on the fact that the increase in the skin conductivity signal value as a function of time must be kept below a certain limit, typically 20 uS/s, if the maximum value is to be considered valid.

Another possible condition for the establishment of a valid peak is that the absolute value of the change in the conductivity signal from a local peak to the following local trough exceeds a predetermined value, such as 0.02 uS.

In addition, a maximum value that occurs at the border of the interval, i.e. at the start point or end point of the interval, should preferably not be considered a valid peak.

Thereby, the purpose is achieved that artefacts that can appear in error situations such as e.g. when electrodes detach from the skin, or other sources of noise or disturbances, do not lead to an erroneous detection of peaks.

The number of peaks calculated in step 37 is stored and used as the current number of peaks during the execution of comparison step 38 below.

The conductivity mean value calculation step 36 and the peak counting step 37 can alternatively be performed in the opposite order, or simultaneously, if desired.

The purpose of the following steps 38-42 is to realize the following functions:

If the number of peaks exceeds a certain limit, but the average conductivity level is unchanged, pain/discomfort shall be detected, the output signal 71 shall be activated, and the output signal 72, if previously activated, shall be reset. If the number of peaks exceeds said limit and the average conductivity level increases, a state of wakefulness is detected, the output signal 72 is activated, and the output signal 71, if previously activated, is reset.

If none of the above conditions are satisfied, the output signal 71 or 72 is reset, if it was previously activated.

In the comparison step 38, the currently calculated number of peaks is compared with a preset limit value. The applicant's tests have shown that a suitable limit value is 0.1 peaks per sec., i.e. two peaks per 20 sec. Alternatively, other values may be determined from clinical tests, to further optimize the performance and reliability of the output indications.

If the current number of peaks is equal to or greater than the preset threshold value (output indicated by Y), the process continues to decision step 40. If, on the other hand, the current number of peaks is less than the preset threshold value (output indicated by N), the process continues at step 39.

In step 39, both output signals 71 or 72 are reset to zero, if either of them had previously been activated.

In the comparison step 40, the current average conductivity value is compared with the previous average conductivity value. If the . current average conductivity value is less than or equal to the previous average conductivity value (output indicated by N), the condition of pain/discomfort should be indicated, and the process continues to step 41. If, on the other hand, the current average conductivity value is greater (output indicated by Y ), the state of wakefulness should be indicated, and the process continues to step 42.

In step 41, the state of pain/discomfort is indicated. The processing unit 53 activates the first output signal 71, which indicates a pain-discomfort state, via the interface circuit 61, and a pain-discomfort message is indicated on the display unit 8 using the interface circuit 81. If the second output signal 72 was previously activated, this is reset. The process then continues at update step 43.

In step 42, the state of wakefulness is indicated. The processing unit 53 activates the second output signal 72, which indicates an alert state, via the interface circuit 61, and an alert message is indicated on the display unit 8 using the interface circuit 81. If the first output signal 71 was previously activated, this is reset. The process then continues at update step 43. In update step 43, the current average conductivity value is stored as the previous average conductivity value. The process is then repeated from step 35.

The process can be interrupted or terminated by an operator device (not shown) or by a command input from the communication port 56.

A first improvement of the method illustrated in fig. 2 will be described in the following: 1 the comparison step 38 in fig. 2, the currently calculated number of peaks is compared with a preset limit value. Even more reliable results can be obtained for the indications of pain/discomfort and alertness if this comparison also depends on the condition that the current number of peaks is greater than the previous number of peaks.

To perform this extended comparison, a further step 34 is to be performed after step 33, where the number of fluctuation peaks in the conductivity signal throughout the initial period is calculated. This calculation is performed in the same manner as described with reference to step 37. The initial number of peaks is stored and used as the "previous number of peaks" in the first execution of the extended comparison step 38.

The comparison stage 38 must also be modified. In the modified comparison step 38, the current number of peaks is compared with the preset limit value and with the previous number of peaks. If the current number of peaks is greater than both the limit value and the previous number of peaks, the process continues to comparison step 40. If, on the other hand, the number of peaks is less than or equal to the limit value or the previous number of peaks, or both, the process continues to step 39.

The update step 43 must also be modified. In the modified update step 43, the current number of peaks is stored as the previous number of peaks. In addition, the current average conductivity value is stored as the previous average conductivity value.

A second improvement to the embodiment illustrated in FIG. 2 will be described in the following: In the embodiment in fig. 2, a time series is first collected and then analyzed. As an advantageous alternative, data acquisition and analysis are performed as separate, independent processes, simultaneously executed by the processing unit 53.

A data collection process is then carried out which virtually continuously updates part of the memory 55 with the last, for example, 20 sec. of skin conductance signal values.

An analysis process is initiated, for example, every 1 second. This process will analyze the last eg 20 seconds of skin conductance data collected by the concurrently executed data collection process. All process steps 35-43 are performed by the analysis process, while the initial process steps 32 and 33 are performed in advance, as initial steps.

This solution leads to an even faster and more reliable indication of pain/discomfort and alertness, compared to the simpler method described with reference to fig. 2.

Fig. 3a-c are three graphs each illustrating a time series of a

skin conductance measurement signal (vertical) versus time (horizontal) for a sedated patient, who is not exposed to either wakefulness stimuli or pain/discomfort stimuli.

Fig. 3a is a graph illustrating a skin conductance signal which is essentially stationary.

A first time interval of approximately 20 sec. is indicated at 301a, and a second time interval of approximately 20 sec. is indicated by 302a.

Assume that the method in accordance with the embodiment described with reference to fig. 2 is applied to this signal, with the initial time series corresponding to the time interval indicated at 301a, and the next time series corresponding to the time interval indicated at 302a. The preset limit value is two peaks per 20 sec. In the second time interval 302a, the number of peaks will be calculated to zero. Then the current number of peaks will be less than the preset limit value. Accordingly, the process will continue to step 39, i.e. both the pain/discomfort state signal and the alertness signal will be reset. The monitoring process will then be repeated, based on the time interval 302a as the previous time interval and a subsequent time interval (not illustrated) as the current time interval.

Fig. 3b is a graph illustrating a time series of skin conductance measurements for a patient whose skin conductance is steadily decreasing.

A first time interval of approximately 20 sec. is indicated at 301b, and a second time interval of approximately 20 sec. is indicated at 302b.

Now assume that the method in accordance with the embodiment described with reference to fig. 2 is applied to this signal, with the initial time series corresponding to the time interval indicated at 301b, and the next time series corresponding to the time interval indicated at 302b. In the second time interval 302b, the number of peaks will be calculated as zero. The number of peaks will then be recognized as less than the preset limit of 2 per 20 sec. The current number of peaks will then be less than the preset limit value. Accordingly, the process continues to step 39, i.e., both the pain-A-discomfort state signal and the alertness signal are reset. The monitoring process will then be repeated, based on time interval 302b as the previous time interval and a subsequent time interval (not illustrated) as the current time interval.

Fig. 3c is a graph illustrating a time series of skin conductivity measurements for a patient whose skin conductivity is stationary increasing.

A first time interval of approximately 20 sec. is indicated at 301c, and a second time interval of approximately 20 sec. is indicated at 302c.

It must be assumed that the method in accordance with the embodiment described with reference to fig. 2 is applied to this signal, with the initial time series corresponding to the time interval indicated at 301c, and the next time series corresponding to the time interval indicated at 302c.

The apparent fluctuations in the signal are due to noise with a relatively small size (less than 0.02 uS). Provided that step 37 is performed with the additional condition that the absolute value of the change in the conductivity signal from a local peak to the following local trough must exceed the predetermined value of 0.02 uS for a peak to be considered valid, the apparent peaks will not be considered valid peaks.

In the second time interval 302c, the number of peaks will thus be calculated to zero. Then the number of peaks is recognized as below the preset limit value of 2 per 20 sec. The current number of peaks will then be less than the preset limit value. Accordingly, the process continues to step 39, i.e., both the pain/discomfort state signal and the alertness signal are reset. The monitoring process will then be repeated.

Fig. 4 is a graph illustrating a time series of skin conductance measurements for a sedated patient exposed to pain-displeasure stimuli.

A first time interval of approximately 20 sec. is indicated at 401, and a second time interval of approximately 20 sec. is indicated at 402. Two subsequent peaks among several peaks in the skin conductance of the needle are indicated at 403 and 404.

Now assume that the method in accordance with the embodiment described with reference to fig. 2 is applied to this signal, with the initial time series corresponding to the time interval indicated at 401, and the next time series corresponding to the time interval indicated at 402.1 the second time interval 402 the number of peaks will be calculated as 10. Then the number of peaks is recognized as equal to or greater than the preset limit. Accordingly, the comparison step 40 will be executed.

Furthermore, the mean skin conductance value through the first time interval 401 will be calculated as approximately 8.3 microsiemens, and the mean skin conductance value through the second time interval 402 will be calculated as approximately 8.2 microsiemens. Then no increase in the mean conductivity value will be recognized in the comparison step 40. Thus the process enters step 41, which means that the first output signal 71 is activated, and a state of pain/discomfort in the patient is indicated.

Then the process proceeds to step 43, ie, the monitoring process is repeated, based on the time interval 402 as the previous time interval and a subsequent time interval (not shown) as the current time interval.

Fig. 5 is a graph illustrating a time series of skin conductance measurements for a sedated patient exposed to wakefulness stimuli.

A first time interval of approximately 20 sec. is indicated at 501, and a second time interval of approximately 20 sec. is indicated at 502. Two peaks in the skin conductance signals are indicated at 503 and 504.

Now assume that the method in accordance with the embodiment described with reference to fig. 2 is applied to this signal, with the initial time series corresponding to the first time interval indicated at 501, and the next time series corresponding to the second time interval indicated at 502. In the second time interval 502, the number of peaks will be calculated as 2.1 the comparison step 38 recognizes the number of peaks that greater than or equal to the preset limit. Accordingly, the comparison step 40 will be executed.

The mean skin conductance value through the first time interval 501 will be calculated as about 2.4 microsiemens, and the mean skin conductance value through the second time interval 502 will be calculated as about 3 microsiemens.

Accordingly, an increase in the average conductivity value is recognized in the comparison step 40. Thus, step 42 is executed and the second output signal 72 is activated, which means that a state of wakefulness in the patient is indicated. Then, the process proceeds to step 43, i.e., the monitoring process is repeated, based on the time interval 502 as the previous time interval and a subsequent time interval (not illustrated) as the current time interval.

The above description and drawings present a specific embodiment of the invention, with the addition of some alternatives. However, it will be obvious to a person skilled in the art that other, alternative embodiments exist and which fall within the scope of the present invention. E.g. the skin conductance signal can be measured using a DC method rather than the specifically described AC method. The use of skin resistance instead of skin conductivity as a measurement signal will of course lead to equivalent results, if the inverse nature of these variables is taken into account. Although the detection of peaks is specified in the detailed description, those skilled in the art will recognize that the same result will appear if troughs or minimum points are detected in a similar manner.

Furthermore, when a patient is exposed to induction of anaesthesia, the number of peaks will decrease along with a decrease in the average conductance level.

The inventive concept is thus not limited to the exemplary embodiments described above. The scope of the invention is instead set out in the following patent claims.

Claims (13)

1. Method for monitoring the autonomic nervous system of a sedated patient, comprising - providing a skin conductivity signal measured at an area (2) of the patient's skin, - calculating characteristics of said skin conductivity signal, - determining (41) a first output signal (71) ) to indicate a state of pain/discomfort in the patient, based on said characteristics of said skin conductivity signal, characterized in that said characteristics of the skin conductivity signal include the average value of the skin conductivity signal over a first time interval and the average value of the skin conductivity signal over a second time interval, and the number of fluctuation peaks or - bottoms for the skin conductivity signal over said second time interval, and - that the method further comprises the step of determining (42) a second output signal (72) to indicate a state of wakefulness in the patient, based on said characteristics of said skin conductivity signal, including comparing the number of fluctuation peaks r or troughs over said second time interval with a limit value, and if the number of peaks or troughs is higher than the limit value, to compare the average conductivity value over the second interval with the average conductivity value over the first interval, and if the second average conductivity value is the largest, to determine the second output signal (72) to indicate the state of wakefulness of the patient. 2. Procedure in accordance with claim 1, where said step of determining the first output signal (71) comprises - comparing the number of fluctuation peaks or troughs over said second time interval with a limit value, and if the number of peaks or troughs is higher than the limit value, - to compare the average conductivity value over the second interval with the average conductivity value over the first interval, and if the second mean conductivity value is not the largest, determining the first output signal (71) to indicate the state of pain/discomfort in the patient. 3. Procedure in accordance with one of claims 1-2, where the first and second intervals have the same duration, e.g. 20 seconds. 4. Procedure in accordance with one of claims 1-3, where the starting point for the second interval is 0.5 to 5 seconds after the starting point for the first interval. 5. Procedure in accordance with claim 4, where the starting point for the second interval is about 1 second after the starting point for the first interval. 6. Procedure in accordance with one of claims 1-3, where the starting point for the second interval essentially coincides with the ending point for the first interval. 7. Apparatus for monitoring the autonomic nervous system of a sedated patient, comprising - measuring equipment (3, 4, 51, 52) for providing a skin conductivity signal measured at an area (2) of the patient's skin, - a data store (54, 55) to store the measured signal values at discrete times, - a control unit (5), arranged for to calculate characteristics for said skin conductivity signal, determining (41) a first output signal (71) to indicate a state of pain/discomfort in the patient, based on said characteristics of said skin conductance signal, characterized in that said characteristics include the average value of the skin conductivity signal over a first time interval and the average value of the skin conductivity signal over a second time interval, and the number of fluctuation peaks or troughs for the skin conductivity signal over said second time interval, and - that the control unit (5) is further arranged to determine ( 42) a second output signal (72) to indicate a state of wakefulness in the patient, based on said characteristics of said skin conductivity signal, including comparing the number of fluctuation peaks or troughs over said second time interval with a limit value, and if the number of peaks or troughs is higher than the threshold value, comparing the average conductivity value over the second interval with the average conductivity value over the first interval, and if the second average conductivity value is the greater, determining the second output signal (72) to indicate the state of wakefulness of the patient. 8. Apparatus in accordance with claim 7, where the control unit (5) is arranged to determine the first output signal (71) by performing the steps - comparing the number of fluctuation peaks or troughs over said second time interval with a limit value, and if the number of peaks or troughs is higher than the limit value, - to compare the average conductivity value over the second interval with the average conductivity value over the first interval, and if the second conductivity mean value is not the largest, - determining the first output signal (71) to indicate the state of pain/discomfort in the patient. 9. Apparatus in accordance with one of claims 7-8, where the first and second intervals have the same duration, e.g. 20 seconds. 10. Apparatus in accordance with one of claims 7-9, where the starting point for the second interval is 0.5 to 5 seconds after the starting point for the first interval. 11. Apparatus in accordance with claim 10, where the starting point for the second interval is about 1 second after the starting point for the first interval. 12. Apparatus in accordance with one of claims 7-9, where the starting point for the second interval essentially coincides with the ending point for the first interval. 13. Apparatus in accordance with one of claims 7-12, further comprising a device for supplying medication to the patient, controlled by the first output signal 71 and/or the second output signal 72.